1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition for use in a pressure sensor for use at high temperatures, a resonator, and the like.
2. Background Art
At present, piezoelectric ceramics are widely used not only in the fields of conventional electronic machines and devices such as resonators and filters but also in products using charges and displacements such as sensors and actuators. Generally, the material therefor has been selected from ferroelectric materials having a perovskite structure such as lead zirconate titanate ("PZT" hereinafter) or lead titanate ("PT" hereinafter). These materials give excellent piezoelectric characteristics when a third component or additive is incorporated. Since, however, most of them as a practical composition have a Curie point of approximately 200.degree. C. to 400.degree. C., they become ordinary dielectric materials at temperatures higher than the above and lose piezoelectric characteristics, so that they can no longer be used at ultrahigh temperatures for a nuclear reactor and the like. Further, since the above materials contain approximately 60 to 70% by weight of lead oxide (PbO), they are undesirable in the ecological point of view and in view of the prevention of environmental pollution.
As a piezoelectric ceramic containing no lead oxide, BaTiO.sub.3 having a perovskite structure similarly to the above materials is well known. Further, JP-A-9-100156 discloses a composition of (BiNa)TiO.sub.3 -NaNbO.sub.3 which has a perovskite structure similarly to the above.
Of piezoelectric ceramic materials having the above compositions:
1 BaTiO.sub.3 has piezoelectric characteristic (electromechanical coupling coefficient) which is good to a great extent, while it has a Curie point of as low as 120.degree. C., so that it is environmentally extremely limited in use. PA1 2 (BiNa)TiO.sub.3 -NaNbO.sub.3 disclosed in JP-A-9-100156 has a large and excellent electromechanical coupling coefficient, while its Curie temperature is 370.degree. C. or lower, so that it cannot be used at a temperature higher than the above. PA1 the solid solution of the above formula satisfying 2.4&lt;(1-x)a+xq&lt;3.1, PA1 0&lt;b&lt;1.1, PA1 0.9&lt;(1-x)c+xr&lt;2.1, and PA1 0&lt;p&lt;0.6.